1. Field of the Invention
The present invention relates to a photoresist copolymer suitable for the submicrolithography to highly integrate semiconductor devices. More particularly, the present invention relates to a photoresist copolymer consisting of glutarimide derivatives and acrylic acid derivatives, which are of high etch resistance and thermal resistance. Also, the present invention is concerned with a photoresist composition comprising such a copolymer.
2. Description of the Prior Art
Recently, chemical amplification photoresist has been prevailing in semiconductor devices because it was found to be highly sensitive to DUV (Deep Ultra Violet) light, which is now recognized as a light source suitable for accomplishing the high integration of semiconductor devices. Chemical amplification photoresist consists generally of a photoacid generator and a matrix polymer having such a chemical structure which sensitively reacts with acid.
The mechanism of such a photoresist is as follows. When the photoresist is exposed through a mask to a DUV light source, protons are generated by the action of the photoacid generator and then, react with the main or side chain of the matrix polymer. This reaction exceptionally increases the solubility of the copolymer in a developing solution by converting the structure of the copolymer, e.g. by decomposing it, cross-linking it or changing its polarity. Therefore, when being treated with the developing solution, the copolymer is dissolved at exposed regions whereas it remains undissolved at un-exposed regions, leaving the shape of the mask as a positive image on a substrate.
There is a relation between a resolution and the wavelength of a light source in photolithography, as shown in the following formula: EQU R=k.times..lambda./NA
wherein R is a resolution; k is a process constant; .lambda. is a wavelength of a light source; and NA is a numerical aperture. As implicated, the smaller the wavelength of light source, the finer the pattern formed.
Accordingly, a research has been directed to find new light sources suitable to improve the resolution. In result, a DUV light was developed as a light source for the integration of semiconductor devices into 1 giga or higher scale. Examples of the DUV light source in current use include krypton fluoride excimer laser (hereinafter referred to as "KrF") and argon fluoride excimer laser (hereinafter referred to as "ArF") which are 248 nm and 193 nm in wavelength, respectively. The latter light source is used for 4G or higher scale DRAM. According to the finding of this new light source, suitable photoresist films should be developed.
In general, a photoresist is required to be of high etch resistance and thermal resistance. In order to improve the etch resistance, aromatic resins are usually sought out. However, because the aromatic resins show high absorbance at short wavelengths, the photoacid generator is illuminated with a reduced amount of light energy and thus, has a reduced opportunity to produce protons.
In the case of using a short wavelength light source, such as ArF, poly(methylmethacrylate) (PMMA) resins, rather than aromatic resins, are used. PMMA resins, however, are difficult to apply in practice because of their poor etch resistance and thermal resistance.